Splitting of the combustion air flow is a well established technique for minimising the emissions of nitrogen oxides (NO, NO2, and N2O, collectively referred to as NOx) that arise from burning any fossil fuel. Burners designed to minimise NOx emissions are known as Low-NOx burners. The invention particularly relates to low nitrogen oxide burners which split combustion air into inner and outer streams, for example so as to stabilize flame and also reduce NOx emissions.
In pulverised coal combustion without NOx control measures, most of the NOx is produced by the oxidation of the organically bound nitrogen in the fuel (so-called fuel NOx), with a lesser amount deriving from atmospheric nitrogen which is oxidised at high temperature (so-called thermal NOx). Controlling the mixing of the combustion air with the fuel leads to the creation of conditions which favour reactions leading to N2 in preference to NOx. These conditions are created in a low NOx burner by aerodynamic means. Typically the combustion air is split into two or more discrete streams, some of which may have a tangential velocity component imposed by means of spin vanes (known as swirl).
The combustion air split may be external to the burner at the windbox, but is more typically undertaken within the burner itself. In each case separate dampers are used to regulate the air flow to each stream within the burner. Such dampers can suffer from a number of difficulties; they may have a non-linear flow response to the damper position, the flow may not be responsive to movement in damper position making control difficult, and some arrangements of damper are difficult to adjust manually. Sleeve type dampers are often favoured, as they are relatively low cost components and avoid some of the manual adjustment difficulties.
Generally, and especially in the case of low NOx burners, the burner assembly comprises of a series of concentrically arranged pipes to supply the fuel and combustion air. To facilitate the flow split between the combustion air streams, and optionally to regulate the air flowrate, internal dampers are an integral feature of fossil fuel burners.
The design of the moveable mechanical components in a fossil fuel burner is an important aspect of the burner design; it can impact on the operation and performance of the burner, the pressure loss of the air flow through the burner, the cost of fabricating the burner, and the ease of maintaining the burner once it is installed.
Generally each air stream within a burner is independently regulated by an individual damper device, though in some burner designs there may be no regulation of one or more of the air streams. This gives rise to a number of issues, including in a typical case one or more of the following.    1) The desired air flow split can be achieved at a number of different damper positions, and this can include settings where the dampers restrict the flow more than is necessary to regulate flow split. As a result such systems are prone to increased pressure drop, leading to increased operational costs (electrical power consumption by the fans used to supply the combustion air).    2) The requirement for multiple dampers leads to increased mechanical complexity, and hence increased fabrication cost and maintenance cost.    3) Operation of one damper impacts the flow to all air streams supplied from the windbox, and hence requires adjustment of the other dampers, leading to greater difficulty in establishing the optimum burner operating settings and increased commissioning cost.